Script Manager Explanation or example

Script Manager Explanation or example

Can somebody explain me in what form add javascript snippet. I have tried with/without script tag, with/without window.onload = function(). No success.
I'm not really a coder. I just want to add some class to element with classname, something like
var v = document.getElementsByClassName("class 1");
v.className += "class 2";
I have undestood that it doesn't work with jquery. Only javascript.
BTW I'm working with Drupal 9. Perhaps it's the problem